Just a small note: 99% of your cards have nice artworks the other 1% doesnt matter youre doing everything right here. That is tho just the artworks not cards.
Much of the appeal of tcgs be it online and offline has to do with how to present your game entities and i feel you fucked up big time here. Remember the feeling of nostalgia when you see a card you played with and you just cant help but think wow this thing hell yeah i remember that! Heres your issue while the artworks on their own are great n stuff the whole picture you actually see in game is … bland it turns into a green/blue/yellow/red piece of blob with text on it and nice borders.
The problem being that the borders are the same color as the artwork you put on it. You could check for yourself with photoshop change the colors of the card while leaving the artwork untouched and youll quickly notice that your cards become much more distinguishable and unique and get a much better shot at engraving themselves in your mind.
Of course this is up to “taste” but id say the general perception is too much of one color is bad (i mean, wearing an orange shirt SURE wearing orange shoes and pants to go with it you look like a dumbass).
If id say theres one visual improvement you should absolutely consider it would be recoloring the cards, something akin to weapons in hs could work neutral main color, colored details indicating faction (looking at how itd be too impractical to change all the arts color to not suit the card color).
Anyhow have a nice one! Love the game 
Edit: To elaborate, looking at the multi color cards you already have that effect but apart from them there are some examples of cards which i find pretty good examples of cards which i find pretty good the way it is:
Wild Avenger, Earthcraft, Eredon, Thyrian (somewhat still feeld kinda meh), Tritron Chef/Banquet, Groundshaker, Boulder Thrower, Wind Wisp (Hell yeah! if it was played… )
